As the semiconductor device market continues to grow and become more competitive, the equipment and processes for manufacturing semiconductor devices are likewise maturing. To keep pace with competition, there is a need for manufacturing processes to be as efficient as possible. Reducing the amount of time, energy, and materials used for manufacture may increase manufacturing efficiency. Accordingly, there has been an expanding effort to reduce the amount of resources used in the manufacturing process. In addition, as semiconductor feature dimensions become ever smaller, it is becoming increasingly difficult to accurately control these dimensions within a narrow range in an efficient and highly repeatable manner.
As part of this effort to increase manufacturing efficiency while maintaining control of feature size, control mechanisms, known in the field as advanced process control (APC) mechanisms, allow various portions of the manufacturing process to be measured and adjusted.
APC mechanisms are often embodied as devices known as integrated metrology modules (IMMs), which are essentially sensors and data processors used in conjunction with various steps in the manufacturing processes. These IMMs take specified measurements during the manufacturing process to determine whether certain errors occur during a manufacturing step. The results of these measurements, which may be fed backward or forward in the manufacturing process, are used as a basis to adjust other aspects of manufacturing in an attempt to compensate for such errors.
Such APC mechanisms also typically measure a send-ahead (SAHD) wafer. A wafer is a planar piece of semiconductor material, typically disc-shaped, that is the substrate on (and in) which semiconductor devices are formed. A SAHD wafer is, in essence, a sacrificial wafer that is used to test-run at least a portion of the manufacturing process so that the manufacturing steps may be adjusted prior to sending future wafers intended for actual marketable semiconductor devices. A SAHD wafer is used for each “run” of wafers, which is typically a group of about twenty or so wafers, although a run may be any number of wafers. In other words, for each run, at least one wafer may be wasted, thereby reducing efficiency. This reduced efficiency is especially problematic where, as is typically the case, the profit margin for the end product is small.